This invention relates to the control of downhole equipment attached to a drill string by the transmission of commands from the surface of the earth. More particularly, the invention relates to a down link communication system for directional drilling and measurement-while-drilling (MWD) systems wherein commands are transmitted as defined rotations or rotation rates of the drill string implemented at the surface, sensed downhole by a gyroscopic sensor, and subsequently transmitted to the controlled downhole equipment.
Borehole drilling technology has advanced significantly during the past two decades. Advances have been particularly significant in the drilling of oil and gas wells. In offshore operations, it is not uncommon to drill dozens of boreholes from a single drilling platform location, where each borehole is directed to a specific target location. This practice is commonly referred to as "directional drilling", and requires that borehole paths be deviated from the vertical direction. The drilling of severely deviated and even horizontal wells has become a common practice used to maximize hydrocarbon production from a single borehole. Measures of geophysical parameters of formations penetrated by the drill bit are now made simultaneously with the drilling operation. These MWD measurements are used to "steer" the drill bit within the formation or formations of interest, and the technique is often referred to as "geosteering".
All of the above techniques require some type of communication between downhole equipment attached to the drill string and personnel or equipment at the surface of the earth. An "up link" communication system provides a means for transmitting downhole sensor response data to the surface. A "down link" communication system from the surface to the downhole equipment is used to control downhole equipment which, in turn, controls the path of the drilled borehole. Down link commands are also used to activate and deactivate various MWD sensors while the borehole is being drilled. During borehole workover operations, various types of downhole or bottom equipment are activated by down link signals from the surface. A down link communication or telemetry system suitable for all of these applications is set forth in this disclosure.
Most modem drilling systems use the circulation of drilling fluid or drilling "mud" as a method for removing drill cuttings, cooling and lubricating the drill bit, and controlling pressures of penetrated formations by providing a hydrostatic head within the borehole. Drilling mud is pumped downward through a string of drill pipe, exits the "drill string" at the bit, and returns to the surface for recirculation through the drill stringborehole annulus. Many prior art telemetry systems have used the mud column within the borehole as a physical channel for communicating with downhole equipment, as well as a physical channel for transmitting downhole sensor data to the surface. Stated briefly, the pressure generated by the mud pump is modulated in order to generate a pressure pattern in the mud column. For up link communication, modulation occurs downhole and the induced pressure pattern is sensed at the surface by a pressure sensor at the surface of the earth and subsequently decoded. For down link communication, modulation occurs at the surface and the pressure pattern is sensed by a pressure sensor or a flow sensor located within the downhole equipment and decoded downhole. This communication technique is not reliable, because variations in the mud column pressure can also be induced by changing environmental conditions such as varying pressures within penetrated formations.
U.S. Pat. No. 3,967,680 discloses a down link communication which utilizes controlled drill pipe rotation and controlled drilling mud flow rate changes to carry out various down hole operations. Commands are defined as a function of the rate of rotation of the drill string. The rate of rotation is sensed downhole by a device comprising a pair of rotatable fly weights attached to a lever apparatus, where the fly weight axis of rotation is coincident with the axis of rotation of the drill string. An increase in drill string rotation rate urges the fly weights outward due to an increase in centrifugal force, and a decrease in drill string rotation rate results in an inward movement of the fly weights due to a decrease in centrifugal force. The radial extension of the fly weights is, therefore, an indication of the rate of drill string rotation. The fly weights cooperate with a spring and actuator apparatus to perform defined downhole operations based upon the rate of rotation of the drill string. The operations are further defined by the mud pressure which is used to power the activator. The entire fly weight and actuator system is mechanically complex, and can be used only to sense the rate of drill string rotation, and not to sense incremental rotations of the drill string. Significant measurement error can also be expected in highly deviated boreholes and at slow drill string rotation rates due to the force of gravity perturbing the centrifugal force acting upon the fly weights.
Deviated wells are often drilled with a drilling system comprising a turbine or "mud motor" attached to the bottom of the drill string. The drill bit is attached to, and can be rotated by, the mud motor which is powered by mud pressure generated by the mud pump. The mud motor can be deactivated and the drill bit can be rotated by rotating the drill string. A deviated subsection or "bent sub" is positioned immediately uphole from the mud motor. When the direction of the bit path is to be changed, the drill bit is stopped and the entire downhole drilling assembly is redirected azimuthally by a controlled rotation of usually a few degrees of the drill string at the surface. The mud motor is again started, and the borehole is drilled in the new direction. U.S. Pat. No. 4,647,853 discloses a system for detecting the rate of rotation of a downhole turbine using a triaxial magnetometer, which is usually a "standard" component carried by deviated drilling systems and which is used in defining the location and orientation of the downhole drilling assembly. A powerful permanent magnet is mounted on the uphole end of the turbine drive shaft, with the magnetic moment of the magnet perpendicular to the axis of the turbine shaft. As the turbine shaft rotates, this turbine mounted magnet superimposes a rotating magnet field on the earth's magnetic field in the vicinity of the turbine. This superimposed rotating field constitutes a mud motor tachometer signal, which is sensed and separated from the response of the system's existing magnetometer. The signal defines the rotation rate of the mud motor turbine, and not the rate of rotation of the drill string. Furthermore, the system can be used only in "open" boreholes, since the magnetometer response is meaningless in boreholes cased with steel casing. The system is also insensitive to any type of incremental rotation.
U.S. Pat. No. 4,763,258 discloses methods and apparatus for telemetering while drilling by changing drill string rotation angle or drill string rotation rate or rotation "speed". The magnitude of an incremental rotation of the drill string is related to an arbitrary downhole function, such as the activation of a specific downhole sensor. The incremental rotation is sensed by a downhole inclinometer and magnetometer, which are normally carried by a deviated hole downhole drilling system to define the orientation and location of the downhole equipment. The outputs of the inclinometer and the magnetometer cooperate with a downhole microprocessor, which sends the signal to execute the sensed command. In another embodiment, the rate of drill string rotation is related to an arbitrary downhole function. The rate of rotation is again sensed by the magnetometer and inclinometer, and these outputs are converted to the defined command by means of the microprocessor. The system requires a three axis magnetometer and a three axis inclinometer. Although normally available with deviated drilling systems, this equipment might not be included in a "standard" package for other downhole operations as, for example, workovers. The response of the magnetometer can be affected by geophysical properties of the formation being penetrated by the drill bit. In addition, the technique is limited to use in open boreholes since the response of the magnetometer is meaningless in boreholes cased with the normal steel casing pipe.
A primary object of the present invention is to provide a down link communication system for operating downhole equipment which does not require a mud circulation system and which does not rely upon a mud column as a physical channel of communication.
Another object of the invention is to provide a down link communication system which can be operated in open boreholes and in boreholes cased with conventional steel casing.
Yet another object of the invention is to provide a stand-alone down link communication system which is applicable to numerous borehole operations, and which is not limited to use in MWD or drilling operations by requiring other downhole components utilized in MWD and/or directional drilling operations.
Still another object of the invention is to provide a cost effective down link communication system which maximizes the use of commercially available parts and minimizes the use of special, expensive, high maintenance components.
Another object of the invention is to provide an accurate down link communication system which incorporates methods for checking error associated with telemetered data.
Yet another object of the invention is to provide a telemetry system which is unaffected by geophysical properties of formations penetrated by the borehole.
There are other objects and applications of the present invention that will become apparent in the following disclosure.